Monday, April 6, 2009

I think today we'll talk about the high and low pressure ranges on the auto a/c system using R-134 refrigerant.First you must understand that a lot of different factors can change the pressure of the freon considerably so I'll give you ranges that the pressure should by in on certain temperature days.On a 70 F degree day the low side should be between 25 and 45 psi and the high side should be in the 175 to 225 range.On a 100 Degree day the low side could go as high as 60 and the high side as high as 275.Again these are typical ranges and other factors could change them.I don't usually get too concerned as long as the high side stays under 300 psi.Okay now what if the pressures aren't within range?

What to look for:

High side pressure too high

Condenser fan not running. If this is an electric fan make sure its on any time the a/c is on

Condenser coil dirty or air flow across coil blocked. Clean coil and check for air flow

Non condense able gases in system. Air or moisture has gotten into the system at some point. Sometimes the system can be purged on the high side to help this problem. In most cases the system should be pulled down and vacuumed. The dryer should be changed as well.

Blockage in the freon flow. Dryer clogged, expansion valve bad or out of adjustment.

System overcharged. Too much freon in the system allows no room for expansion. Pull some of the freon back out and recheck pressures.

Low side pressure too low.

Low in freon. High pressure side will be reading low as well. Find the leak and recharge the system

Evaporator clogged and freezing up.

Evaporator fan not running

Duct controls not functioning properly

Freon flow restricted. Clogged dryer or expansion valve bad or out of adjustment. High side pressure should be reading higher than normal.

you might want to add temp + pressure + RPM. That makes a difference. The faster the RPM the better it pulls the temp down. You cant expect to get the same temp at idle at a red light as you do going on the interstate at 70 mph or 2500 RPM.